1. Field of the Invention
The present invention relates to an optical pickup device including a light source, an optical element, and a supporting member for supporting the optical element through an adhesive, and particularly to an optical pickup device using a blue-violet laser as the light source.
2. Description of Related Art
Recently, there have been developed and manufactured optical pickup devices for recording or reproducing information on or from various optical discs such as CDs (Compact Discs), DVDs (Digital Versatile Discs), and BDs (Blu-ray Discs).
For example, as shown in JP 2008-305516 A, such an optical pickup device includes a light source (for example, a semiconductor laser), a detector unit (for example, a semiconductor light receiving element), and optical elements (for example, a diffractive lens, a collimating lens, and a relay lens) through which light emitted from the light source and light reflected from an optical disc and guided to the detector unit pass.
The optical elements are fixed, with an adhesive, to the device body and its components such as a lens holder of an objective lens driving device. The adhesive is a resin that can be cured by ultraviolet light (UV-curable resin).
Conventionally, glass has been used as a material for optical elements. Recently, however, in response to a demand for cost reduction in optical pickup devices, plastic has been used as a material not only for a diffractive lens having a diffraction grating formed on its surface but also for optical elements such as a collimating lens and an objective lens.
With an increase in the capacity of optical discs, short-wavelength lasers have been used as light sources. For example, an infrared laser with a center wavelength of 780 nm is used as a light source for CDs. A red laser with a center wavelength of 650 nm is used as a light source for DVDs. A blue-violet laser with a center wavelength of 405 nm is used as a light source for BDs.
In response to a demand for size reduction of the device body and its components such as a lens holder, a UV-curable adhesive can be applied to a limited area, and thus is used in the vicinity of the optical element or on the edge portion thereof. The optical element transmits outgoing light emitted from the light source and guided to the optical disc, and returning light reflected from the optical disc and guided to the detector unit. When the optical element transmits these lights, the adhesive used in the vicinity of the optical element or the edge portion thereof may be irradiated with part of the lights. The adhesive also may be irradiated with unnecessary stray light components that have not been guided to the detector unit.
Typically, a UV-curable adhesive has an absorption spectrum shown in FIG. 6, in which a slight absorption is observed in the wavelength range of blue-violet laser light. The UV-curable adhesive is cured when it is exposed to ultraviolet irradiation to fix the optical element. In this case, part of a photopolymerization initiator remains unreacted. Therefore, if a blue-violet laser is used as a light source, when the adhesive is exposed to the light from the light source, the remaining unreacted photopolymerization initiator in the adhesive causes a further curing reaction. Accordingly, in an optical pickup device using a blue-violet laser as a light source, as the use of the optical device increases, that is, as the continuous blue-violet laser irradiation time increases, the curing of the adhesive proceeds further and further.
As the adhesive is cured further in this way, its qualities are changed (for example, it shrinks or becomes deformed). The cure shrinkage and deformation of the adhesive generate stresses, which change the position, posture, shape, etc. of the optical element. If the optical element is made of plastic, since the plastic optical element has a lower modulus of elasticity than a glass optical element, the change in the shape (aberration) of the plastic optical element increases accordingly.
Therefore, in an optical pickup device including such a plastic optical element and using a blue-violet laser as a light source, the optical element is more affected by the further curing of the adhesive when it is irradiated with the light from the light source. Therefore, in this optical pickup device, aberrations, such as astigmatism and coma, in the optical element occur as the use of the optical device increases, resulting in a difficulty in maintaining the quality of the optical device for a long period of time.
On the other hand, as an optical device in which an adhesive is not irradiated with light emitted from a light source, JP 2009-10232 A discloses an optical device for an exposure apparatus including an optical element, and a supporting member for supporting the optical element through an adhesive. In this optical device, a light blocking film is formed on the surface of the optical element. FIG. 7 shows the specific configuration of this optical device.
In FIG. 7, a lens is fixed to a supporting member through an adhesive, light blocking films are provided in the vicinity of the bonding portion of the lens, and moreover, a light blocking member is provided on the light irradiation side of the supporting member so as to cover the adhesive. JP 2009-10232 A describes that as the light blocking films, a resin containing a pigment or a material containing Ni or Si can be used, and a material having both a transmittance and a reflectance of 0.1% or less can be used.